Because there must be something before there can be one of it. Two cannot exist without something that’s the same in essential attributes as the other thing. Then there can be two things but there is no “two”, only two of something.
Existence of something is necessary before a number is needed. Without something there would have been no need to refer to the attribute of the thing, no need for a number to be used at all.
All of the exotic sounding information about number theory must start at the beginning. The thing that exists. Just thinking about one as a word means nothing because there must first be something about which to think. The primacy of thought is a philosophic system that assigns characteristics and attributes to the thoughts of a person. The primacy of existence analyses something and assigns identifies attributes and characteristics to something that’s real. A thought is real but only to the person who has it. An existent is real whether or not someone knows it. Number theory is important to identify real things which really do exist.
Arithmetic is not a theory about numbers. It’s a theory about how real things can be used but in a computational way. That’s why two oranges cannot be counted with two apples. the apples and the oranges exist apart from each other so they cannot be put in the same group. Even when it’s said that two apples and two oranges can be added to get four things, that’s impossible to do. Two apples are vastly different than two oranges so there no way to combine them. Any time a number theory seeks to explain that two apples and two oranges can be combined, that theory is false. True number theories are only useful with real things, things that exist. Of course that means past, present and future existents.
A great example of correct number theory is the European method of never referring to a first floor in a hotel. They explain the first floor is itself. There is no need to add a word and say “first floor”. They also call the ground level floor “ground level”. That leads to good thinking because it’s based on correct ideas.